(A) Neutrophils were depleted by intraperitoneal injection of 500 μg anti-Ly6G antibody (1A8, BioXCell) into pregnant mice at E4.5. Ly6G FACS analysis of whole blood from untreated and anti-Ly6G–injected pregnant mice at day E8.5. (B) Ly6G (neutrophils) immunohistochemical staining of sagittal paraffin sections of embryos within the uterus. Pregnant females were either treated with the isotype antibody (2A3, BioXCell) as a negative control or with anti-Ly6G for neutrophil depletion. Representative images of control (n = 6) and Cmas^–/– (n = 4) embryos. (C) Mean fetal size of control and Cmas^–/– embryos from anti-Ly6G–treated mother mice, as measured by the sum of the areas of the amniotic cavity, exocoelomic cavity, ectoplacental cavity, and embryo proper (μm²/10⁵) (control, n = 6; Cmas^–/–, n = 4). For a schematic overview of the measured areas, see Supplemental Figure 5. Error bars indicate SD. Statistical analyses were performed by Student’s t test (**P < 0.01). (D) Immunohistochemical detection of cytokeratin-8 to visualize trophoblast cells on sagittal paraffin sections of E8.5 uteri from Ly6G-treated mother mice. Representative images of control (n = 6) and Cmas^–/– (n = 4) embryos. (E) Collagen IV indirect immunofluorescence staining on sagittal paraffin sections of uteri at E8.5 from Ly6G-treated mother mice to visualize RM (arrow) and parietal endoderm (arrowheads). Representative images of control (n = 6) and Cmas^–/–(n = 4) embryos. Nuclei stained with DAPI are shown in white.